The present invention relates to a system and method for recharging a rechargeable battery. More specifically, the present invention relates to a battery recharging system and method using an indirect contact battery temperature sensor, where the system is part of an electronic device, such as a two-way wireless communication device, cellular telephones, pagers, etc.
Such devices may be powered through use of rechargeable batteries. The rechargeable batteries used in these devices typically include NiCd or NiMH cells. There are two conventional ways to determine when these types of cells have finished being recharged. The first way is to compare the temperature of the battery to the ambient temperature. The second way is to detect a drop in voltage that occurs near the end of a charging cycle.
Using the first method, prior art devices monitor the temperature of the cell in comparison to the ambient temperature in another section of the electronic device. When the cell is fully charged, the cell temperature may rise 20 degrees centigrade or more above the ambient temperature, depending on the charging rate. This rise in temperature occurs because once the battery is fully charged, the electrical energy being input into the battery can no longer be converted to chemical energy and stored in the battery. The electrical energy being input into the battery is instead converted to heat energy. Consequently, the battery becomes quite warm, and remains warm as long as the charging continues.
This first method is essentially limited to custom designed battery packs in which a temperature sensor can be placed in physical contact with the cell package. In products that accept AA size (IEC LR6) rechargeable batteries, the ability to place a temperature sensor against the battery is limited since the battery diameters vary and a temperature sensor placed on the wall of the battery compartment does not always contact the battery adequately for thermal conduction.
Using the second method, when the cell is charged from a totally dead state, the cell voltage rapidly rises in the first portion of charging, and then levels out until the cell is almost completely full. At the end of the charging (normally defined as between 110 and 125% of charge) the voltage rises slightly, then dips. This rise and dip near the end of the charging cycle is called "negative-delta-V" because the voltage of the cell has a negative slope, even though energy is being added to it and is caused by various chemical processes inside the cell. Many charging systems detect this drop in voltage and use it to terminate the charging cycle. New NiMH cells exhibit very little negative-delta-V and therefore it is very hard to detect. In addition, if the battery is recharged at the same time a dynamic load is connected, then any increase in the load may cause a decrease in the battery voltage. This would look like a negative-delta-V condition and would cause charging to terminate early.